This invention relates to a surface plasmon resonance sensor or probe used in biochemical, chemical, biological or other applications.
Surface plasmon resonance (xe2x80x9cSPRxe2x80x9d) is an optical phenomenon caused by the interaction between light or other electromagnetic radiation and several different types of materials, usually comprising a dielectric material and a conductive material arranged in a multi-layer stack of thin films. Technical details describing this optical phenomenon are set forth in various publications, such as one by Schwotzer, et al., titled Fiber Optic. Sensor for Adsorption Studies Using Surface Plasmon Resonance, vol. 2508, Institute of Radio Engineering and Electronics, pp. 324-33, and patents, including U.S. Pat. Nos. 4,997,278 and 5,485,277, each of which documents are incorporated herein by reference.
Basically, however, SPR is an optical phenomenon that occurs when light is shined at a certain angle into a prism that has upon one surface a thin coating comprising one or more conductive or dielectric layers. If the light is shined into the prism at a particular xe2x80x9ccritical angle,xe2x80x9d the light may totally internally reflect within the prism so that it does not escape that side of the prism. The critical angle depends upon the characteristics of the prism, the layer(s) or the environment surrounding the entire structure. For instance, an everyday example of total internal reflection occurs when you peer into a clear glass of water. As you change the angle and orientation of the glass relative to your line of sight, at some point you will see the sides of the glass turn opaque or silver. Even though you can normally see through the water in the glass, at the critical angle at which the sides turn silver or opaque, the light is totally internally reflected within the glass and water therein because of the different refractive indices of the water, glass and surrounding air.
Light that totally internally reflects within a coated prism forms an electromagnetic wave that propagates along the conductive (i.e., metal) layer boundary. This wave is known as a surface plasmon. The surface plasmon wave is optically excited at the interface between a conductor or semiconductor, e.g., a metal surface and a dielectric. The optical excitation takes place by an evanescent field, created when light undergoes total internal reflection, for example, off the base of a prism. This evanescent field penetrates the metal and excites a surface plasmon wave where the metal meets the dielectric.
It takes energy to create the surface plasmon. The energy forming the surface plasmon is removed, at a specific frequency or wavelength, from the light that hits the interface between the prism and its coating. Thus, the resulting reflected light beam lacks the removed energy. If you examined the energy in the reflected beam across a spectrum of frequencies or wavelengths, you would see a dip or drop in the energy at a particular xe2x80x9cplasmon resonance wavelength,xe2x80x9d which is the wavelength at which the surface plasmon removes energy from the reflected beam. The plasmon resonance wavelength is determined by a number of factors, including: the thickness, composition and number of the conductive or dielectric layers, as well as the incidence angle of the light upon the substrate, and the interaction between the metal or dielectric layers and the ambient environment.
This surface plasmon resonance (xe2x80x9cSPRxe2x80x9d) phenomenon can be and has been used to create sensors that sense the presence of certain chemical, biological or biochemical agents. For instance, incorporating a particular dielectric or other transducing layer whose permittivity and/or thickness varies in response to chemicals (analytes) of interest results in a sensor whose normal SPR frequency changes with that variation. By analyzing the degree and type of the change, one can determine the presence and/or quantity of a particular analyte of interest. SPR sensors are generally based on bulk optical components (prisms, polarizers, etc.) that yield high quality resonances but which are very difficult to miniaturize into suitable probes for remote sensor applications.
Examples of such SPR sensors are described in U.S. Pat. Nos. 5,485,277 or 4,977,278 or in the Schwotzer, et al. publication cited above. These sensors usually work by shining a collimated light through focusing lenses into a prism or other high refractive index material and then detecting and analyzing the reflected light with a spectrum analyzer. A baseline surface plasmon resonance frequency is found for the particular sensing medium, such as a metallic or other layer of material, coating the prism. The addition of an analyte to the sensing medium changes the SPR frequency. A detector analyzes the reflected light to detect the new SPR frequency. By comparing the new versus baseline frequency, the analyte and/or its quantity can be identified and detected. Such sensors are usually bulky and difficult to keep properly calibrated during their employment.
Indeed, U.S. Pat. No. 4,997,278 to Finland, et al. itself recognizes that one problem with sensors that use a prism or the like is that slight movements of the prism or light source result in changes to the incidence angle, which in turn changes the SPR frequency. That means that the changes to SPR frequency detected by the sensor will be rendered inaccurate or less accurate since variables (e.g., the movement) other than just the presence and amount of analyte will alter the relationship between the baseline SPR frequency and the SPR frequency obtained with the analyte. Prior sensors, including Finland, et al.""s, also use a variety of optics in order to collimate, focus and guide the incident and reflected light beams. These optics contribute to the bulkiness of the probe sensors, rendering them both more expensive to build or maintain and less versatile during use.
Another problem with these conventional SPR probes is that they normally use only one area upon the surface of the probe as the sensing medium. For instance, the Finland, et al. patent applies a sensing layer to the rear, planar surface of an optically transmissive component formed of a slide in contact with a cylindrical lens. Finland, et al., then passes a collimated beam of non-coherent light through the hemispherical portion of the lens so that the light impacts upon the flat surface of the slide upon which the metallic film has been formed. Finland describes the non-coherent light that it shines upon the sensing medium as a fan or cone shaped beam of light. Finland proposes that the advantage of such a fan or cone shaped beam of light is that the range of angles of incidence of the light at the intersection point spans the angle which excites a SPR in the film. Although this allows Finland, et al. to use several beams of light to impact the sensing medium, Finland, et al.""s probes are like prior probes that still use only one portion as a sensing medium and still require monochromatic operation (e.g., use of a particular single wavelength of light) and focusing optics.
The present invention is an SPR probe that has a substrate with a generally curved reflecting surface. In the present invention, light is input through the substrate to the generally curved reflecting surface where it interacts with one or, optionally, multiple, sensing areas coated with the same or different sensing mediums. By causing the light first to impact against the curved reflecting surface, the light may be reflected from a first impact area to a second, third, etc. impact area. That is because the radius of curvature of the substrate causes the light incident upon the first impact area to reflect to another portion of the substrate with the same incident angle. Thus, for each of the impacts of light on the different portions of the substrate the incident angle remain constant. The number of reflections, and thus the number of light impact and potential sensing areas, can be adjusted by modifying the shape, size and curvature of the curved reflecting surface, as well as the location at which the light enters the substrate. In a preferred embodiment, the curved reflecting surface may be formed as a hemisphere, a shape that is fairly easy to grind to the quality levels required for optical materials.
The invention also involves forming an SPR probe without the need of collimating and/or focusing optics. A probe may be formed by mounting a substrate, such as a substrate with a hemispherical or generally curved surface, on a mandrell or other holding device. Fiber optic lines or light waveguides may be threaded through the mandrell so that an input line provides the incident light that shines through the substrate to impact a generally curved surface coated with a sensing medium. For instance, if a curved or hemispherical substrate is used, the substrate may have a generally planar portion coupled directly to a fiber optic line in order for light to enter the planar portion without distortion and impact the curved reflecting surface. A return fiber optic line may be set within the mandrell at a position that intersects the point at which the curved reflecting surface ultimately reflects the incident light back toward, and through, another generally planar portion of substrate into the return fiber. This structure obviates the need for lenses to focus light upon the substrate. In essence, the substrate itself acts as a lens that focuses the diverging cone of incoming light upon the curved portion of substrate at a stable incident angle. Use of the probe fashioned in this manner, where focusing is acheived by reflection alone, additionally permits the use of white light. Such achromatic operation both eliminates the need for collimating optics and increases the overall range of the available wavelengths for the incident light.
In another embodiment of this invention, the sensing medium coating the curved reflecting surface of the probe may be either a continuous film or an optical diffraction grating. As described in U.S. Pat. Nos. 5,502,560 and 5,610,708 to Anderson, et al., each of which documents are incorporated herein by reference, the reflection spectrum from an optical diffraction grating in contact with sample analytes can be analyzed for intensity and phase modulations that help determine the bulk dielectric properties of the solution or gas in contact with the optical diffraction grating. For instance, the optical diffraction grating can be formed as part of the substrate itself or in the film coating the substrate.
In another embodiment of the invention, the curved reflecting surface of the probe forms a sensor head that may be detachable, facilitating the reuse and reconfiguration of the probe to detect other analytes of interest. In other words, the user can easily swap out a used probe sensor head for an unused or different type of sensor head without having to purchase or replace the entire probe. In one embodiment of this invention, the probe may be an attenuated total reflectance (xe2x80x9cATRxe2x80x9d) probe, such as the hemispherical ATR probe available from Equitech Int""l Corporation of Aiken, S.C.
Additionally, the present invention can be implemented in an overall system that comprises the probe, with its fiber optic input and detection lines as well as a curved or hemispherical substrate. The probe may be coupled to an input light source for providing light, whether collimated, non-collimated, single or multiple wavelength. The output fiber line of the probe may be coupled to a detector. For instance, a spectrum analyzer may be coupled to the output fiber line in order to analyze the reflected light. A microprocessor-based system may be used automatically to calculate the baseline SPR wavelength and the SPR wavelength following interaction of the sensing medium with the analyte of interest. A display may be coupled to the microprocessor for depicting the baseline and changed SPR wavelengths or frequencies. The microprocessor may incorporate software or couple to a DSP chip for performing filtering or other digital signal processing upon the information the detector provides.
The use of the probe of this invention offers multiple advantages over prior prism or fiber optic based probes. First, the curvature of the probe""s reflecting surface and the intimate connection of the fiber to the substrate itself provides a more stable incident angle than a prism substrate than a separate lens and prism substrate configuration. Second, the intimate fiber coupling with the sensor head obviates the need for lenses. Third, the hemisphere or curved sensor portion of the probe may be easily removed, thereby facilitating the reuse and reconfiguration of the probe. Fourth, the probe permits white light, achromatic operation and increases the overall range of the available wavelengths. Another aspect of this invention involves the multiple applications for which it may be deployed. For instance, many current SPR probes are not suitable for field deployment because of their bulk, complexity and fragility. The SPR probe fashioned according to this invention, however, may be used not only for gas phase moisture sensing but also for biological agent detection and commercial gas phase sensing operations in the field.
By way of example, testing and comparison of a sensor fashioned according to this invention against a reference electronic humidity sensor has demonstrated that the SPR sensor of this invention offers substantially better moisture detection performance. Current electronic humidity sensors rely on diffusion through a polymer and take on the order of one or more seconds to respond to humidity changes. The accuracy of these sensors are on the order of one percent relative humidity. The SPR sensor of this invention, however, has a high dynamic range and allows for gas phase kinetic studies to be performed in-situ, thus providing a process-ready sensor that generates better results than currently available sensors. This is particularly critical for certain operations, such as chemical processing that involves volatile or explosive chemicals in which safely tracking relative humidity with a non-electrical sensor is important. By way of example, one embodiment of this invention coats the SPR substrate with a thin metal film that supports the surface plasmon resonance wave, as well as a thin layer of silicon dioxide, SiO2, which acts to sorb water molecules. The process of water adsorption onto and into the sillica layer changes the optical constants and thickness of the material in contact with the metal layer due to the addition of monolayers of water and ocndensed water in the silica pores. The change in optical constants effectively changes the resonance condition and thus the position of the resonance in the optical reflection spectrum. This change in position can be read out using an optical spectrometer and compared against calibrated results to detect the amount of water present. In other words, the sensor can be used to detect relative humidity or dewpoint.
Thus, using the SPR probe of the present invention, faster, more accurate, and more reliable relative humidity data will be available in applications ranging from moisture sensing and drying ovens to air monitoring with a hand-held, portable analyzer.
The present invention accordingly aims to achieve at least one, more or combinations of the following objectives:
To provide an SPR substrate where input light is incident upon multiple sensing surfaces of the substrate.
To provide an SPR substrate that has at least a generally curved reflecting surface with one or multiple sensing locations upon the generally curved reflecting surface and with which light entering the substrate interacts.
To provide an SPR substrate that can be used with non-collimated light.
To provide an SPR substrate that can be used without one or more lenses for focusing or collimating the entering or reflected light.
To provide an SPR substrate upon which multiple reactions can take place by modifying the sensing medium at each point of reflection.
To provide an SPR substrate with a sensing medium formed as a continuous film or with a diffraction grating.
To provide a sensor probe that has a replaceable sensor head having a substrate with a generally curved reflecting surface.
To provide a probe that couples to a system for automatically detecting the SPR points of various analytes of interest.
To provide an SPR probe that may be used in various applications, including applications in which the SPR probe tracts the water vapor concentration in the ambient environment.
Other objects, features and advantages of this invention will become apparent from the rest of this document, including the Figures.